The black hole effect in perimetry.

Light-emitting diodes (LEDs), mounted in drilled holes in the perimeter bowl, are used as stimuli in several automated perimeters. A concern is that these "black holes" might interrupt the otherwise uniform background illumination and cause inconsistent test results. A Dicon perimeter was modified by covering some of the LEDs with diffusing plastic. One eye of 41 normal volunteers was tested repetitively within the central 5 degrees of the visual field at the same 12 locations with both covered and uncovered LED stimuli. Higher variances of multiple threshold determinations were observed, significant at the 0.0005 level, when testing was done with uncovered LEDs. On average, the black hole effect contributed 0.8 dB to short-term fluctuation. The black hole effect is probably of minor clinical importance except in exacting quantitative perimetry.